This invention relates generally to a ground leakage current responsive circuit interrupter and, more particularly, to such an interrupter that distinguishes between dangerous fault currents and acceptable residual leakage currents.
All electrical equipment powered by standard power sources of over 35 volts is capable of supplying sufficient electrical energy to give severe and lethal shocks. A hazardous electrical shock occurs at any time more than five milliamps of current is conducted through a person's body. The risk of shock is dependent upon the magnitude of the power sources involved and is substantially increased by wetness in the area of equipment usage. Most usually, a hazardous shock occurs when a person is in contact with a ground source such as a body of water or wet ground or grounded metal pipes, frames, etc., and makes additional contact with a potential source capable of supplying hazardous electrical current. A source can include, for example, electrical equipment that establishes a substantial current leakage path or a hot power line with which a person inadvertently comes into contact. Such contact can result from the careless removal of protective insulator covers, bypassing safety interlocks or tampering with safety grounding. Other potential sources of lethal currents, not commonly considered, are tools and appliances that are accidentally dropped into a body of water. Regardless of how well they are insulated, motors require ventilation openings that permit water access and result in the application of line potential to the water body.
Protection against electrical shock is generally provided by ground fault circuit interrupters designed to activate a tripping circuit in response to leakage currents of about five milliamps. These devices customarily include a differential transformer having a single core with two or more primary windings, each winding being connected in one of the conductors supplying the load to be protected. The primary windings are so wound that they produce in the core magnetic fields which oppose one another. A secondary winding is also provided on the core and suitable circuitry is provided such that, when a voltage develops across the secondary winding, it will open a circuit interrupter in the supply conductors. Under normal conditions, the currents in the supply conductors and the primary windings are equal. Thus, the resultant flux produced in the core by the primary windings is zero and no voltage is developed across the secondary winding. However, when a separate path to ground is established--as, for example, through a human body--the currents in the primary windings will immediately become unbalanced. A resultant flux is then produced in the transformer core which induces a voltage across the secondary winding. This voltage thereupon actuates the tripping circuit which opens the circuit interrupter and disconnects the load from the power supply.
Ground fault circuit interrupters, however, exhibit a number of serious disadvantages. One problem stems from the presence of residual leakage current resulting from electrical equipment design that produces a continuous electrical path between a power line and earth ground. Large electrical equipment often exhibits residual leakage exceeding five milliamps and consequently cannot be equipped with conventional ground fault protective equipment. Another problem frequently occurs on construction sites on which a single interrupter is shared by a plurality of electrical tools. The total residual leakage of the multiple tools can easily exceed a protective unit's threshold and cause "nuisance tripping". This latter factor discourages the use of a single ground fault interrupter to protect an entire house, apartment, construction site, etc., and because of the cost involved in providing multiple protective interrupters such facilities generally are unprotected.
The object of this invention, therefore, is to provide an improved ground leakage current interrupter that can distinguish between hazardous fault current and acceptable residual leakage currents.